Section 2 Electricity and Magnetism - South Windsor Public Schools

Electricity and Magnetism

Sect2Elect&Mag Glencoe Science Electricity and Magnetism 2002 ISBN 0-07-825620-8 Page 1

Section 2 Electricity and Magnetism

As You Read

What You'll Learn Describe the relationship between electricity and magnetism.

Explain how electricity can produce motion.

Explain how motion can produce electricity.

Vocabulary electromagnet generator

motor alternating current

aurora transformer

Why It's Important The electric current that comes from your wall socket is available because of magnetism.

Moving Charge Creates a Magnet In the first section, you learned that a moving electric charge makes a magnetic

field. You also learned that electrons moving around the nucleus of an atom produce tiny

magnetic fields, and in some materials these magnetic fields can group together into

domains, making a magnet. In addition to their movement in atoms, electrons can be

made to move in other ways. When an electric current is created, electrons flow in a wire.

These moving electrons also produce a magnetic field. Figure 9A shows the magnetic

field produced around a current-carrying wire.

Look at the magnetic field lines around the coils of wire in Figure 9B. The

magnetic fields around each coil of wire add together to form a stronger magnetic field

inside the coil. When the coils are wrapped around an iron core, the magnetic field of the

coils magnetizes the iron. The iron then becomes a magnet, which adds to the strength of

the magnetic field inside the coil. A current-carrying wire wrapped around an iron core is

called an electromagnet, as shown in Figure 9C.

Figure 9 A current-carrying

wire produces a magnetic field.

9A. Iron particles show the

magnetic field lines around a

current-carrying wire.

9B. When a wire is wrapped in

a coil, the field inside the coil

is made stronger.

9C. An iron core inside the coils

increases the magnetic field because

the core becomes magnetized.



Using Electromagnets The magnetic field of an

electro-magnet is turned on or off when the electric

current is turned on or off. By changing the current,

the strength and direction of the magnetic field of an

electromagnet can be changed. This has led to a

number of practical uses for electromagnets. A

doorbell, as shown in Figure 10, is a familiar use of

an electromagnet. When you press the button by the

door, you close a switch in a circuit that includes an

electromagnet. The magnet attracts an iron bar

attached to a hammer. The hammer strikes the bell.

When the hammer strikes the bell, the hammer has

moved far enough to open the circuit again. The

electromagnet loses its magnetic field, and a spring

pulls the iron bar and hammer back into place. This

movement closes the circuit, and the cycle begins

again. The hammer strikes the bell several times

each second.

Figure 10 An electric doorbell uses an electromagnet. Each time the electromagnet is turned on, the hammer strikes the

bell. How is the electromagnet turned off?

D When the

hammer strikes

the bell, the

circuit is open,

and the

electromagnet

is turned off.

C The

electromagnet

attracts the hammer

that strikes the bell.

B When the circuit is

closed, an electromagnet

is turned on.

E A spring

pulls the hammer

back, closing the

circuit and

starting the cycle

over.

A Pressing the button closes

the circuit.

Bell

Mini-Lab Assembling an Electromagnet

Procedure

1. Wrap a wire around a 16-penny

steel nail ten times. Connect one end

of the wire to a D-cell battery, as

shown in Figure 9C. Leave the other

end loose until you use the

electromagnet. WARNING: When

current is flowing in the wire, it can

become hot over time.

2. Connect the wire. Observe how

many paper clips you can pick up

with the magnet.

3. Disconnect the wire and rewrap the

nail with 20 coils. Connect the wire

and observe how many paper clips

you can pick up. Disconnect the wire

again.

Analysis

1. How many paper clips did you

pick up each time? Did more coils

make the electro-magnet stronger or

weaker?

2. Graph the number of coils versus

number of paper clips attracted.

Predict how many paper clips would

be picked up with five coils of wire.

Check your prediction.



Some gauges, such as the gas gauge in a car, use a galvanometer to move the

gauge pointer. Figure 11 shows how a galvanometer makes a pointer move. Ammeters

and voltmeters used to measure current and voltage in electric circuits also use

galvanometers, as shown in Figure 11.

VISUALIZING VOLTMETERS AND AMMETERS

The gas gauge in a car uses a device called a galvanometer to make the needle of

the gauge move. Galvanometers are also used in other measuring devices. A voltmeter

uses a galvanometer to measure the voltage in an electric circuit. An ammeter uses a

galvanometer to measure electric current. Multimeters, like those on the right, can be

used as an ammeter or voltmeter by turning a

switch.

Figure 11

11A A galvanometer has a pointer attached

to a coil that can rotate between the poles of a

permanent magnet. When a current flows through

the coil, it becomes an electromagnet. Attraction

and repulsion between the magnetic poles of the

electromagnet and the poles of the permanent

magnet makes the coil rotate. The amount of

rotation depends on the amount of current in the

coil.

11B To measure the current in a circuit

an ammeter is used. An ammeter

contains a galvanometer and has low

resistance. To measure current, an

ammeter is connected in series in the

circuit, so all the current in the circuit

flows through it. The greater the current

in the circuit, the more the needle

moves.

11C To measure the voltage in a circuit

a voltmeter is used. A voltmeter also

contains a galvanometer and has high

resistance. To measure voltage, a

voltmeter is connected in parallel in the

circuit, so almost no current flows

through it. The higher the voltage in the

circuit, the more the needle moves.



Magnets Push Currents Look around for electric appliances that produce motion, such as a fan. How does

the electric energy entering the fan trans-form into

the kinetic energy of the moving fan blades? Recall

that current-carrying wires pro-duce a magnetic

field. This magnetic field behaves the same way as

the magnetic field that a magnet produces. Two

current-carrying wires can attract each other as if

they were two magnets, as shown in Figure 12.

Electric Motor just as two magnets exert a force

on each other, a magnet and a current-carrying wire

exert forces on each other. The magnetic field

around a current-carrying wire will cause it to be

pushed or pulled by a magnet, depending on the

direction the current is flowing in the wire. This

behavior can be used to convert the electric energy

carried by the current into kinetic energy of the

moving wire, as shown in Figure 13A. Any device

that converts electric energy into kinetic energy is a

motor. To keep a motor running, the current-

carrying wire is formed into a loop so the magnetic

field can force the wire to spin continually, as shown

in Figure 13B.

Figure 12

Two wires carrying current in

the same direction attract each

other, just as unlike magnetic

poles do.

Figure 13

An electric motor uses the

inter-action between

electricity and magnetism to

transform electric energy into

kinetic energy.

13A A magnetic field like the one shown will

push a current-carrying wire upward.

13B The magnetic field exerts a force on the wire loop,

causing it to spin as long as current flows in the loop.



Pushing on Currents in Space Every minute the Sun is emitting charged particles

that stream through the solar system like an enormous electric current. Just like a current-

carrying wire is pushed or pulled by a magnetic field, Earth's magnetic field pushes and

pulls on the electric current generated by the Sun. This causes most of the charged

particles in this current to be deflected so they never strike Earth, as shown in Figure 14.

As a result, living things on Earth are protected from damage that might be caused by

these charged particles. At the same time, the solar current pushes on Earth's

magnetosphere so it is stretched away from the Sun.

The Aurora Most of the charged particles from

the Sun are deflected by Earth's magnetosphere.

However, some charged particles spiral along

Earth's magnetic field lines. These field

lines come together at the magnetic

poles, so these particles eventually enter

Earth's atmosphere high above Earth's

poles. There they collide with atoms in

the atmosphere. These collisions transfer

energy to the atoms, which then

immediately reemit the energy in the

form of light. The light emitted forms a

display known as the aurora (uh-ROR-

uh), as shown in Figure 15. When the

aurora can be seen in northern latitudes,

it is sometimes called the northern lights.

Figure 15

An aurora is a natural light show that

occurs in the southern and northern skies.

Figure 14 Earth's

magnetosphere

deflects most of the

charged particles

streaming from the

Sun. A few, however,

are trapped and spiral

down toward the

poles of Earth.

Why is the

magnetosphere

stretched away from

the Sun?

Figure 15 An aurora is a natural light show that

occurs in the southern and northern

skies.



A Magnet Pushes on Moving Charge

In an electric motor, a magnetic field turns electricity into motion. A device called a

generator uses a magnetic field to turn motion into electricity. Electric motors and electric

generators both involve conversions between electric energy and kinetic energy. In a

motor, electric energy is changed into kinetic energy. In a generator, kinetic energy is

changed into electric energy. Figure 16 shows how a current can be produced in a wire

that moves in a magnetic field. As the wire moves, the electrons in the wire also move in

the same direction, as shown in Figure 16A. The

magnetic field exerts a force on the moving

electrons that pushes them along the wire, as

shown in Figure 16B, creating an electric current.

Generating Electricity To produce electric

current, the wire is fashioned into a loop, as in

Figure 17. A power source pro-

vides the kinetic energy to spin the

wire loop. With each half turn, the

current in the loop changes direc-

tion. This causes the current to

alternate from positive to negative.

Such a current is called an alter-

nating current (AC). In the United

States, electric currents change

from positive to negative to

positive 60 times each second.

Figure 17 In a generator, a power source spins a

loop in a magnetic field. Every half

turn, the current will reverse direction.

This type of generator supplies

alternating current to the light bulb.

Figure 16

When a wire is made to move through a magnetic field, an electric current can be

produced in the wire.

16A If a wire is pulled through a magnetic

field, the electrons in the wire also move

downward.

16B The magnetic field then exerts a force

on the moving electrons, causing them to

move along the wire.



Types of Current A battery produces direct current instead of alternating current. In a

direct current (DC) electrons flow in one direction. In an alternating current, electrons

change their direction of movement many times each second. Some generators are built to

produce direct current instead of alternating current.

Reading Check What type of currents can be produced by a generator?

Power Plants Electric generators are used to produce electric energy all over the

world. Small generators can produce energy for one household, and large generators in

electric power plants can provide electric energy for thousands of homes. Different

energy sources such as gas, coal, and water are used to provide the kinetic energy to rotate

coils of wire in a magnetic field. Coal-burning power plants, like the one pictured in

Figure 18, are the most common. More than half of the electric energy generated by

power plants in the United States comes from burning coal.

Voltage The electric energy produced at a power plant

is carried to your home in wires. Recall that voltage is a

measure of how much energy that electric charges in a

current are carrying. The electric transmission lines from

electric power plants transmit electric energy at a high

voltage of about 700,000 V. Transmitting electric energy

at low voltage is less efficient because more electric

energy is converted into heat in the wires. How-ever, high

voltage is not safe for use in homes and businesses. A

device is needed to reduce the voltage.

Science Online

Research Visit the

Glencoe Science Web

site at

science.glencoe.com

for more information

about the different types

of power plants used in

your region of the

country. Communicate

to your class what you

learn.

Figure 18 Coal-burning power plants

supply much of the

electric energy for the

world.



Changing Voltage A transformer is a device that changes the voltage of an alternating current with little loss

of energy. Transformers are used to increase the voltage before transmitting an electric

cur-rent through the power lines. Other transformers are used to decrease the voltage to

the level needed for home or industrial use. Such a power system is shown in Figure 19.

Transformers also play a role in power adaptors. For battery-operated devices, a power

adaptor must change the 120 V from the wall outlet to a voltage that matches the device's

batteries.

Reading Check What does a transformer do?

A transformer has two coils of wire wrapped around an iron core, as shown in Figure 20.

One coil is connected to an alternating current source. The current creates a magnetic

field in the iron core, just like in an electromagnet. Because the current is alternating, the

magnetic field also alternates, switching direction when the current does. This alternating

magnetic field in the

core then causes an

alternating current in

the other wire coil.

Figure 19

Electricity travels

from a generator to

your home.

Figure 20 A transformer can increase or decrease

voltage. The ratio of input coils to

output coils equals the ratio of input

voltage to out-put voltage. If the input

voltage here is 60 V, what is the output

voltage?



The Transformer Ratio Whether a transformer increases; or decreases the input

voltage depends on the number of coils on each side of the transformer. The ratio of the

number of coils on the input side to the number of coils on the output side is the same as

the ratio of the input voltage to the output voltage. For the transformer in Figure 20 the

ratio of the number of coils on the input side to the number of coils on the output side is

three to nine, or one to three. If the input voltage is 60 V, the output voltage will be 180

V.

In a transformer the voltage is greater on the side with more coils. If the number

of coils on the input side is greater than the number of coils on the output side, the

voltage is decreased. If the number of coils on the input side is less than the number on

the output side, the voltage is increased.

Superconductors Electric current can flow easily through materials, such as metals, that are

electrical conductors. However, even in conductors, there is some resistance to this flow

and heat is produced as electrons collide with atoms in the material.

Unlike an electrical conductor, a material known as a super-conductor has no resistance

to the flow of electrons. Superconductors are formed when certain materials are cooled to

low temperatures. For example, aluminum becomes a superconductor at about -2720C.

When an electric current flows through

a superconductor, no heat is produced and no electric energy is converted into heat.

Superconductors and Magnets Super-conductors also have other unusual

properties. For example, a magnet is repelled by a superconductor. As the magnet gets

close to the super-conductor, the superconductor creates a magnetic field that is opposite

to the field of the magnet. The field created by the superconductor can cause the magnet

to float above it, as shown in Figure 21.

Figure 21 A small magnet floats

above a superconductor.

The magnet causes the

superconductor to produce

a magnetic field that repels

the magnet.



Using Superconductors Large electric currents can flow through electromagnets

made from superconducting wire and can produce extremely strong magnetic fields. The

particle accelerator shown in Figure 22 uses more than 1,000 superconducting

electromagnets to help accelerate subatomic particles to nearly the speed of light. Other uses for superconductors are being developed. Trans-mission lines made from a

superconductor could transmit electric power over long distances without having any

electric energy converted to heat. It also may be possible to construct extremely fast

computers using microchips made from super-conductor materials.

Magnetic Resonance Imaging A method called magnetic resonance imaging, or MRI, uses

magnetic fields to create images of the inside of a human

body. MRI images can show if tissue is damaged or

diseased, and can detect the presence of tumors.

Unlike X-ray imaging, which uses X-ray radiation that can

damage tissue, MRI uses a strong magnetic field and radio

waves. The patient is placed inside a machine like the one

shown in Figure 23. Inside the

machine an electromagnet made

from superconductor materials

produces a magnetic field more

than 20,000 times stronger than

Earth's magnetic field.

Figure 22 The particle accelerator at Fermi National

Accelerator Laboratory near Batavia, Illinois,

accelerates atomic particles to nearly the speed of

light. The particles travel in a beam only a few

millimeters in diameter. Magnets made of

superconductors keep the beam moving in a circular

path about 2 km in diameter.

Figure 23 A patient is being placed

inside an MRI machine.

The strong magnetic

field inside the machine

enables images of tissues

inside the patient's body

to be made.



Figure 24

This MRI image shows a side

view of the brain. An MRI scan

can produce images from several

angles, as well as cross sections.

Producing MRI Images About 63

percent of all the atoms in your body are

hydrogen atoms. The nucleus of a hydrogen

atom is a proton, which behaves like a tiny

magnet. The strong magnetic field inside the

MRI tube causes these protons to line up

along the direction of the field. Radio waves

are then applied to the part of the body being

examined. The protons absorb some of the

energy in the radio waves, and change the

direction of their alignment.

When the radio waves are turned off, the

protons emit the energy they absorbed and

realign themselves with the magnetic field.

The amount of energy emitted depends on

the type of tissue in the body. This energy

emitted is detected and a computer uses this

information to form an image, like the one

shown in Figure 24.

Connecting Electricity and Magnetism

Electric forces and magnetic forces are similar in

some ways. Both forces can repel or attract. Like

electric charges repel each other, and like magnetic

poles repel each other. Positive and negative electric charges attract, and north and south

magnetic poles attract.

Electric charges and magnets are connected in another important way. Moving

electric charges produce magnetic fields, and magnetic fields exert forces on moving

electric charges. It is this connection enables electric motors and genera-tors to operate.

Section 2 Assessment 1. What is an electromagnet? How can you make one in the classroom?

2. How does a transformer work?

3. How does a magnetic field affect a current-carrying wire?

4. How does a generator turn motion into electrical energy?

5. Think Critically How is an electric motor similar to an aurora? Use the terms current,

field, and kinetic energy in your answer.

6. Researching Information Research how electricity is generated in your state. Make a

poster showing the fuels that are used. For more help, refer to the Science Skill

Handbook.

7. Calculating Ratios A transformer has ten turns of wire on the input side and 50 turns

of wire on the output side. If the input voltage is 120 V, what will the output voltage be?

For more help, refer to the Math Skill Handbook.

Documents

Section 2 Electricity and Magnetism - South Windsor Public Schools